1. Field of the Invention
The present invention relates to coating solutions for use in forming Bi-based ferroelectric thin films, and ferroelectric thin films, ferroelectric capacitors and ferroelectric memories formed with said coating solutions, as well as processes for production thereof. More particularly, the invention relates to coating solutions that lead to only a small amount of separation (segregation, precipitation) of excessive metallic elements' particles, little leakage current, are excellent in hydrogen heat treatment resistance as well as voltage resistance, are capable of forming dense Bi-based ferroelectric thin films, and have good keeping quality. The invention also relates to ferroelectric thin films, ferroelectric capacitors and ferroelectric memories formed with such coating solutions, as well as processes for the production thereof.
2. Description of Related Art
Thin films of bismuth layer-structured ferroelectrics (BLSF) represented by the general formula (Bi.sub.2 O.sub.2).sup.2+ (A.sub.m-1 B.sub.m O.sub.3m+1).sup.2- [where A is a mono-, di- or trivalent ion (as of Bi, Pb, Ba, Sr, Ca, Na, K or a rare earth element) or combinations of these ions; B is a tetra-, penta- or hexavalent ion (as of a metallic element like Ti, Nb, Ta, W, Mo, Fe, Co or Cr) or combinations of these ions; and m is an integer of 1-5] have recently been found to feature good characteristics such as requiring small coercive field in remanent polarization P-E hysteresis curves and hence experiencing less fatigue as a result of repeated polarization switching. This has spotlighted the potential use of BLSF thin films as materials for the fabrication of semiconductor memories and sensors (T. Takenaka, "Bismuth Layer-Structured Ferroelectrics and Their Grain Orientation" in Report of the Workshop on Applied Electronics Properties, The Japan Society of Applied Physics, pp. 1-8, Nov. 22, 1994; and "Ceramics", vol. 30, No. 6, pp. 499-503, 1995). Among the BLSF thin films so far reported, those of an SrBi.sub.2 Ta.sub.2 O.sub.9 system which are represented by the formula (Bi.sub.2 O.sub.2).sup.2+ (SrTa.sub.2 O.sub.7).sup.2- are of particular interest since the desired characteristics are conspicuous in them.
Such BLSF thin films can be formed by various methods including sputtering, CVD and coated film formation. However, due to the many metal oxides that have to be incorporated as constituents, sputtering and CVD techniques require costly apparatus and considerable difficulties are involved in controlling the composition of ferroelectric thin films at desired levels; hence, these techniques are not suitable for practical applications, particularly on large-diameter substrates. In contrast, the coated film formation technique does not need expensive apparatus and can deposit films at relatively low cost; in addition, they provide ease in controlling the composition of ferroelectric thin films at desired levels. Therefore, the coated film process for the formation of BLSF thin films holds much promise for commercial use.
While several formulations have been proposed for use as coating solutions in the formation of BLSF thin films by the coated film process, two typical cases are those prepared by dissolving carboxylate (e.g., 2-ethylhexanate) of Sr and Bi and alkoxides of Ta in acetate esters (Proceedings of the 12th Ferroelectric materials and their Applications meeting on May 24-27, 1995, Paper presented by Mitsubishi Materials Corporation, 24-TP-11, pp. 57-58; and "Jpn. J. Appl. Phys.", vol. 34, pp. 5096-5099, 1995) and those prepared by dissolving 2-ethylhexanate of Sr, Bi, Ta, Nb, Ti, etc. in xylene to form coating solutions of a metallo-organic decomposition (MOD) type (Proceedings of the 12th Ferroelectric Materials and their Applications meeting on May 24-27, 1995, Paper presented by Olympus Optical Co., Ltd. and Symetrix Corporation, 26-TC-10, pp. 139-140). However, these coating solutions have had various problems. First, the 2-ethylhexanate of the respective metal components has a long-chain (C.sub.8) organic group, so a large portion of the coating solution is occupied by the organic content and there is much loss in the coating weight due to the burning out of the organic content in the process of film formation consisting of the application of the coating solution, baking of the applied coating and crystallization and a porous film will result. In addition, the surface morphology of the coating film is not satisfactory enough to provide for easy application to the fabrication of VLSI devices. Further in addition, in order to form a thin film using the applied coating, the applied coating has to be annealed twice at an elevated temperature of 800.degree. C. to ensure appropriate electrical characteristics thereof, however, this is problematic from the viewpoint of semiconductor fabrication process. It is pointed out that in the conventional BLSF-based coating solutions, high-volatility metals such as Bi are burnt out on forming a thin film, particularly during prebaking and annealing, and therefore electrical characteristics are impaired. Attempt have been made to add in advance high-volatility Bi excessively in a molar amount 1.15-1.3 times as great as the stoichiometric amount to the coating solution in order to compensate the loss of burnout of Bi.
The foregoing "Jpn. J. Appl. Phys." vol. 34 (1995) reports that thin films which contain Sr in an amount of smaller than stoichiometric amount have improved in their film characteristics such as crystallinity.
Even when using a coating solution having a composition out of the theoretical amount (stoichiometric amount) as mentioned above, it was difficult to form a BLSF film of a composition eventually satisfying the stoichiometry since the amount of burnout of metallic elements differ from one another, depending on annealing conditions. Such a BLSF film out of stoichiometry is apt to suffer defects in the film, and therefore excessive metallic elements sometimes separate among particles (grain boundaries) during the growth of the crystallized particles. Particularly, fine particles rich in Bi may separate at grain boundaries and cause an increase in leakage current. When applying a ferroelectric memory processed by using the film above to an actual semiconductor device memory having a very small area, problems such as film fatigue resulting from repeated polarization switching, increase in leakage current, and decrease in hydrogen heat treatment resistance and voltage resistance are posed under the effect of the aforesaid defects and film non-uniformity caused by precipitates.
On the other hand, a rapid heating method for forming a film known as RTP (Rapid Thermal Processing) is now attracting the general attention because of such favorable merits as the possibility of accomplishing a baking treatment in a very short period of time, and hence of reducing damages to the substrate, and of applying heat uniformly to inside and outside of the substrate. The present inventors have confirmed that the foregoing fine particles rich in Bi apparently separate at grain boundaries on forming a film by RTP.
Although it is desirable to form a film from the very beginning of whose composition is closest to the stoichiometric composition, the amount of burnout of Bi and other metallic elements during a formation of a thin film differ from one another due to the conditions of the thin film. It was therefore difficult to form a film having a composition close to the stoichiometric composition in the conventional BLSF-based coating solutions.
The prior art coating solutions have further problems. The long-chained metal carboxylates (metallic soaps) of monobasic acids which are commonly used in the coating solutions are generally slightly soluble in polar solvents and, hence, aromatic solvents such as xylene and toluene are used to prepare the coating solutions. However, the coating solutions using such aromatic solvents have to be stored in glass or metallic containers in order to ensure that the evaporating solvents will not be lost to the ambient atmosphere. On the other hand, glass and metallic containers have the disadvantage that metallic components will dissolve out into the coating solution and this is by no means desirable in the art of semiconductor fabrication which hates the contamination with metallic impurities. Under these circumstances, it is preferred to use polyethylene or polypropylene containers which release only negligible amounts of contaminating metallic impurities, which can be protected against mechanical shocks by simple handling procedures and which are less costly and it is desired to use solvents that will leak out of the plastic containers in negligibly small amounts.
The aforementioned aromatic solvents are also very toxic to humans and subject to increasingly rigorous regulations in the methods of use, management and so forth.
If the long-chained metal carboxylates of monobasic acids are replaced by short-chained metal carboxylates of monobasic acids, little solubility is achieved in practical organic solvents. Lower alkoxides of metals are soluble in several polar solvents but on account of the great tendency to be hydrolyzed with moisture in the air, they have only poor keeping quality and practically acceptable levels of reproduction cannot be achieved in the result of coating operations.
It is the common practice to form a ferroelectric memory by providing a lower electrode/a ferroelectric thin film/an upper electrode on a substrate, and in the preceding or subsequent step of the following aluminum wiring step, form a protective film of SiO.sub.2 or the like (passivation) on the upper electrode. However, it is reported that, in a conventional memory which comprises, for example, Pt/PZT/Pt using Pb(Zr, Ti)O.sub.3 (hereinafter abbreviated as "PZT") as ferroelectric materials, the ferroelectric film deteriorate in its properties during the passivation ("Progress and Production Technique of Ferroelectric Memory/LSI", the 45th VLSI Forum, VLSI Report, Press Journal Co., Ltd., Feb. 28, 1997; Preprint for the 44th Appl. Phys. Related Soc. Meeting, No. 2, pp. 435-436, Mar. 28-31, 1997).
This is attributable to the fact that, because monosilane (SiH.sub.4) is used as a material for forming the passivation film, hydrogen generated from SiH.sub.4 during film forming is dissociated by the catalytic action of Pt of the upper electrode into an atomic-state hydrogen (hydrogen radical) having a strong reducibility, resulting in breakage of the PZT film. This film breakage poses such problems as film peeling between Pt and PZT, deterioration of hysteresis characteristics. A repeated polarization switching also may not be available.
This film breakage is caused by the presence of atomic-state hydrogen having a high reducibility and reduction of the PZT ferroelectric film, which is an oxide film, by the heat treatment. When using a ferroelectric film other than a PZT, a similar film breakage is considered to occur particularly when using Pt for the upper electrode.
Since Bi has been excessively added in the conventional BLSF films, the excess Bi precipitates at grain boundaries, which is easily reduced through a heat treatment in a hydrogen (reducing) atmosphere, and this may cause film deterioration including an increase in leakage current.
When a BLSF film is reduced by the formation of an SiO.sub.2 passivation film prior to forming an aluminum wiring, it is possible to bring the reduced BLSF film back to the normal state by applying a heating treatment in an oxygen atmosphere. When the BLSF film is reduced by the formation of an SiO.sub.2 passivation film after forming an aluminum wiring, the heating temperature is limited due to reflowability of aluminum, and it is difficult to bring the BLSF film back to the normal state.
It is usual to apply an operation known as "sintering" comprising the steps of, after forming the aluminum wiring, performing a heat treatment by the use of a mixed gas of several percent hydrogen and nitrogen or an inert gas such as argon, and improving conductivity on the interface between the aluminum wiring and the electrode. This however causes deterioration of the BLSF film because of the presence of hydrogen having a high reducibility.
It is therefore required to be free from deterioration of the film during formation of the passivation film and in sintering, and to be excellent in hydrogen heat treatment resistance.
Under these circumstances, it has been strongly desired to develop a coating solution that is capable of forming highly dense films of good quality, which lead to little leakage current and are excellent in hydrogen heat treatment resistance and pressure resistance, with minimal coating weight loss due to the burning of the organic content and separation (segregation) of metallic elements in the coating solution and which is soluble in practical organic solvents and which have good keeping quality.
The term "hydrogen heat treatment resistance" of a ferroelectric thin film as used in this specification means deterioration resistance of a ferroelectric thin film against hydrogen having a high reducibility, which occurs during formation of an SiO.sub.2 passivation film or upon sintering treatment after formation of an aluminum wiring.